With the advent of increasing demands for inter-processor sharing of data, it has become desirable to be able to transmit large amounts of data from one processor to another processor over telephone lines. To facilitate the transmission of such large amounts of data, telephone lines have been adapted to carry data communications in digital form. Prior to this adaptation, telephone lines running from customers' premises were designed to carry only low frequency voice communications.
The local access arrangement, which comprises customer premises equipment (CPE) and metallic telephone wires, must be enhanced to accommodate such high speed digital communications such as needed for inter-processor communication. Once such enhancement has been completed, the CPE has access to the facilities of existing all digital switching networks such as the Accunet.RTM. facilities. Recently, the enhancements to the local loop facilities have taken the form of switched channel data service units (DSU) which are installed at the CPE to enable full duplex communication at a high bit rate (i.e., 56 kilobits per second, or a sub-multiple thereof). The communications travel from the customer's data terminal equipment (DTE) through the DSU where they are converted for transmission over four wire telephone lines to the digital switching network.
The digital switching network equipment operates in a synchronous mode, wherein the framing of bytes is maintained throughout the network by a master reference clock. The CPE, however, receives no byte boundary information. Therefore, synchronization problems arise in communications between the network and the CPE. One such problem is the present lack of call progress information when setting up a communication via the digital switching network from a data terminal at one CPE node to a destination CPE node. Call progress information is conventionally obtained in an analog voice communication system by listening over the phone handset to call progress tones. These tones include the dial tone, the line busy signal, the ring back signal, the trunk busy signal and other call progress tones.
A need exists for a similar system for all digital communications between data terminals over a digital network.
Presently, direct digital access office channel units (DDA/OCU), located at telephone company offices, interface the digital switching network with the local loop. The interfacing is accomplished by translating the bit stream from the network (i.e. DS-0 bit stream (64 Kbps)), into a lower bit rate format (i.e., 56 Kbps) for transmission to the CPE. The 8 Kbps differential is used for signalling and network maintenance. The 56 Kbps format stream is then carried over the four wire local loop to the interface switched channel data service unit at the CPE and thence, to the DTE.
Within the digital switching network, certain tones called "call progress tones" (CPT's) are used to indicate the progress of communication during a call setup period. Presently, in communication between the switching network and the DTE, the state of the A and B signalling bits is used to indicate "on hook" or "off hook" conditions. These signalling bits are found in the eighth bit of signalling frames of the network T1 carrier. (See A.T.T. Pub. 43801, "Digital Channel Bank Requirements and Objectives", Nov., 1982.)
When these bits indicate an "on hook" condition, a control mode idle (CMI) bit pattern is continuously transmitted from the DDA/OCU of the network system to the DSU. After the calling party has completed dialing and until the called party answers, the calling party receives only CMI ("on hook") signals. There is no way to determine the call progress because of this characteristic of the network.
This lack of call progress information can be especially troubling as there is no way to distinguish between calls which have failed due to busy conditions, either line or trunk, and calls which are ringing through but which are just not being answered.